1. Field of the Invention
The present invention relates generally to cellular communications. More particularly, the present invention relates to signal strength measurements in cellular communications.
2. Description of the Related Art
In wireless communications, signal strength measurements are used to, for example, detect or predict interference between adjacent frequency bands. This detection or prediction may, in turn, be used to trigger various handovers of User Equipment (UE). (The term “adjacent” in this application includes 2nd adjacent carriers, 3rd adjacent carriers, and optionally includes the guard band separation associated with the carriers in case any one or more of the 2nd adjacent carriers, 3rd adjacent carriers, and/or guard band separation is interfering too.)
Soft handovers are used to gradually transfer a UE connection from one base station to another base station without the user experiencing any interruption in communications. Hard handovers are used to, for example, switch between different frequencies because of interference, load, etc. Although it may not always be apparent to the user, hard handovers typically involve some interruption in the connection between the UE and the base station(s).
The connection between the UE and the radio access network includes an uplink (UL) connection on a first carrier frequency and downlink (DL) connection on a second carrier frequency different than the first carrier frequency. In conventional cellular communications, there are a plurality of predetermined duplex UL-DL pairs in a frequency band. Interference is minimized by providing a relatively small, constant, fixed offset between the UL and DL frequencies of each 1:1 pair. However, in modern communications, the offsets may be dropped, and the DL and UL frequencies need not be arranged into predefined 1:1 pairs or even into a single frequency band. The DL frequencies, the UL frequencies, or both, may differ significantly and may be in different frequency bands in order to, for example, permit more efficient spectrum utilization.
A large variance in the frequencies used for the DL and UL makes it more difficult to detect or predict interferences and, in turn, to conduct timely handovers or avoid falsely triggered handovers. Two frequencies in the same cell can have different degrees of attenuation. There can be additional cell specific uncertainties such as antenna gains, cable losses, leakage (ACLR) from co-sited adjacent carriers, power amplifier rating, base station power settings, etc. Some of the uncertainties can be difficult to determine even when communications are not occuring.
The lack of accuracy in current interference detection and avoidance methods (i.e., about +/−4-9 dB for RSSI measurements—RSSI=received signal strength indicator) inhibit the efficiency and performance of advanced cellular systems. While the accuracy of the signal strength measurements could be increased by increasing the number of UE measurements such an approach could also lead to degradation in service quality, in network capacity or in network coverage. Another way of increasing the accuracy of the signal strength measurements could be to perform calibration measurements when building/upgrading the network (e.g. measuring the cable loss). This approach would on the other hand drastically increase capital expenditures and operational expenditures in network configuration and updating.